Photocatalysts such as titanium oxide have recently become extensively utilized. The use of photocatalysts can realize the decomposition of various harmful substances such as NOx by taking advantage of photoenergy.
Various proposals have hitherto been made on techniques that decompose NOx with photocatalysts (for example, JP H01(1989)-218622A (PTL 1) and JP 2001-162176A (PTL 2)).
In JP H01(1989)-218622A (PTL 1), a photocatalyst and activated carbon as a gas adsorbent are incorporated. Since, however, the activated carbon is not resistant to heat, this composition cannot be utilized after firing.
In JP 2001-162176A (PTL 2), a mixture composed of photocatalyst particles, a weakly acidic oxide such as ZrO2, and a fluororesin binder is fixed onto a glass fiber cloth by heat treatment at 370° C. for 2 min. Since, however, the fluororesin is soft, satisfactory abrasion resistance cannot be obtained.
On the other hand, various proposals have also been made on techniques that form a photocatalyst layer on fired building materials such as tiles (for example, JP S63(1988)-83106A (PTL 3), JP 2000-136370A (PTL 4), and WO 00/06300 pamphlet (PTL 5).
In JP S63(1988)-83106A (PTL 3), photocatalytic tiles are obtained by coating a titanium tetraisopropoxide on a tile surface and firing the assembly at 480° C. for one hr. As far as the present inventors know, according to this method, when ensuring the fixation between a tile base material and a photocatalyst layer is contemplated, only a photocatalyst layer having a small thickness can be formed and, consequently, a satisfactory photoctalytic gas decomposition activity cannot be obtained.
In JP 2000-136370A (PTL 4), photocatalytic tiles are obtained by mixing photocatalytic titanium oxide particles and at least one material selected from the group consisting of alkali metals, alkaline earth metals, zinc, aluminum, platinum, palladium, ruthenium, alumina, zirconia, ceria, and yttria and firing the mixture at 880° C. for one hr. According to this method, the adhesion between the tile base material and the photocatalyst layer is satisfactory by virtue of firing at elevated temperatures of 880° C. or above, and, at the same time, the photocatalytic hydrophilizing function is satisfactory. As far as the present inventors know, however, a sintering reaction of titanium oxide occurs upon firing at 800° C. or above and grain growth of titanium oxide particles occurs, making it impossible to satisfactorily obtain a photocatalytic gas decomposition activity.
In WO 00/06300 (PTL 5), photocatalytic tiles are obtained by coating pohotoctalytic titanium oxide particles and an alkali silicate on a tile base material surface and rapidly heating the coated surface. According to this method, by virtue of heat treatment of the alkali silicate, the adhesion between the tile base material and the photocatalyst layer is satisfactory and, at the same time, the photocatalyst layer surface is relatively smooth. Accordingly, photocatalytic tiles are satisfactory, for example, in photocatalytic hydrophilizing function and photocatalytic antimicrobial function. As far as the present inventors know, no satisfactory photocatalytic gas decomposition activity can be obtained.
Further, various proposals have been made on techniques that, in forming a photocatalyst layer, use Zr (zirconium) materials in addition to photocatalyst particles (for example, WO 99/28393 pamphlet (PTL 6), JP 2009-270040A (PTL 7), and JP 2000-136370A (PTL 4)).
WO 99/28393 (PTL 6) discloses a technique that fixes photocatalytic titanium oxide particles having a specific surface area of 50 to 400 m2/g and at least one Zr compound selected from the group consisting of zirconium oxychloride, zirconium hydroxychloride, zirconium nitrate, zirconium sulfate, zirconium acetate, zirconium ammonium carbonate, zirconium potassium carbonate, zirconium ammonium carbonate, sodium zirconium phosphate, and zirconium propionate on a glass plate at 150° C. Here the Zr compund is utilized as a binder that fixes the photocatalytic titanium oxide particles onto the base material at low temperatures. As described in paragraph 29 of this document to the effect that “in the case of a heat-resistant base material, heat treatment can be carried out at 200 to 400° C. to sinter photocatalytic oxide particles,” the function as the binder is rapidly deteriorated when the temperature exceeds 400° C. Accordingly, when firing is carried out at elevated temperatures of 400° C. or above, satisfactory abrasion resistance cannot be obtained.
JP 2009-270040A (PTL 7) discloses a technique that includes: mixing photocatalytic titanium oxide particles with an acidic sol that includes amorphous Zr—O particles having a particle diameter D50 of 1 to 30 nm dispersed in a dispersion medium, contains an inorganic acid and a carboxylic acid or salt thereof, and uses amorphous Zr—O particles having a zeta potential of not less than 20 mV as a dispersoid; and coating and fixing the mixture on a glass plate at 110° C. Here the amorphous Zr—O particles are utilized as a binder that fixes photocatalytic titanium oxide particles on a base material at low temperatures. When firing is carried out at an elevated temperature above 400° C., Zr—O particles are grown and satisfactory abrasion resistance cannot be obtained.
According to a similar experiment conducted by the present inventors, the Zr material particles could not be dispersed in a coating liquid containing photocatalytic titanium oxide particles and an alkali silicate, described in WO 00/06300 (PTL 5).
JP 2000-136370A (PTL 4) discloses a photocatalyst tile obtained by mixing photocatalytic titanium oxide particles and zirconia particles and firing the mixture at 880° C. for one hr. Here the zirconia particles are incorporated for photocatalytic hydrophilizing function improvement purposes. In this constitution, however, a sintering reaction of titanium oxide occurs upon firing at 800° C. or above and growth of titanium oxide particles occurs, making it impossible to obtain a satisfactory photocatalytic gas decomposition activity. Further, the zirconia particles are also grown and thus do not satisfactorily contribute to an improvement in photocatalytic gas decomposition activity.